The IEEE 802.11n and the draft IEEE 802.11 ac standards include the use of the multiple-input multiple-output (MIMO) technology in wireless digital networks operating in accordance with the IEEE 802.11 family of standards. The availability of multiple antennas necessitated by the introduction of the MIMO technology allows the beamforming technique to be utilized to improve the performance, that is, to increase the data rates and/or to decrease the error rate.
By exploiting radio frequency (RF) conditions between the transmitter and the receiver, a beamforming transmitter (beamformer) is capable of steering RF signal power maxima onto an intended receiver (beamformee) by appropriately weighting data streams transmitted through two or more transmit antennas while transmitting a signal.
The IEEE 802.11n standard includes the technique of transmit beamforming (TxBF), which is to be used in transmitting data between a wireless network device and a single client device at a time, and the draft IEEE 802.11 ac standard specifies the technique of downlink multi-user MIMO (MU-MIMO), through the use of which a wireless access point can transmit separate data on separate spatial streams to multiple client devices simultaneously. The draft IEEE 802.11 ac standard utilizes the beamforming technique at the wireless network device in the MU-MIMO mode to minimize interference among spatial streams intended for different client devices.
An accurate estimate at the beamformer of the RF conditions between the transmitter and the receiver, i.e., the channel state, is critical for the operation of the beamforming technique, whether in a single-user TxBF mode or in a MU-MIMO mode. Explicit feedback is one of the better methods of deriving at the beamformer an estimate of the channel state. The IEEE 802.11n standard includes the explicit feedback technique that can be used with the beamforming technique, and the draft IEEE 802.11ac standard requires explicit feedback when the beamforming technique is utilized.
With explicit feedback, the beamformer transmits a sounding frame including known content, and the beamformee receives the sounding frame, estimates the channel from the frame, generates channel state information and sends the information back to the beamformer. The beamformer then utilizes this information to generate and weight streams of data transmitted through its antennas for the operation of the beamforming technique.
Under certain circumstances, explicit feedback may introduce significant overhead and reduce the throughput available for actual data transmissions. The interval between transmissions of two explicit feedback cycles needs to be very small for a proper operation of MU-MIMO, especially when a 64-quadrature amplitude modulation (64-QAM) or a 256-QAM modulation and coding scheme (MCS) is utilized. For example, a wireless access point with a 3×3 (3 transmit antennas and 3 receive antennas) antenna configuration communicating with three client devices each having a 1×1 antenna configuration on an 80 MHz-wide channel in the MU-MIMO mode has to receive approximately 3000 bytes of feedback information every 10 milliseconds, in addition to the sounding overhead.
Because overhead associated with explicit channel state feedback is one of the critical parameters in deciding whether the beamforming technique should be used and determining the overall system efficiency, it is important to develop methods to minimize overhead without sacrificing RF or system performance.